July 10, 2003
02:00 PM (EDT)

News Release Number: STScI-2003-19

Oldest Known Planet Identified

The full news release story:

Long before our Sun and Earth ever existed, a Jupiter-sized planet
formed around a sun-like star. Now, 13 billion years later, NASA's
Hubble Space Telescope has precisely measured the mass of this
farthest and oldest known planet. The ancient planet has had a
remarkable history because it has wound up in an unlikely, rough
neighborhood. It orbits a peculiar pair of burned-out stars in
the crowded core of a globular star cluster.

The new Hubble findings close a decade of speculation and debate as
to the true nature of this ancient world, which takes a century to
complete each orbit. The planet is 2.5 times the mass of Jupiter.
Its very existence provides tantalizing evidence that the first
planets were formed rapidly, within a billion years of the Big Bang,
leading astronomers to conclude that planets may be very abundant in
the universe.

The planet now lies in the core of the ancient globular star cluster M4,
located 5,600 light-years away in the summer constellation Scorpius.
Globular clusters are deficient in heavier elements because they formed
so early in the universe that heavier elements had not been cooked up
in abundance in the nuclear furnaces of stars. Some astronomers have
therefore argued that globular clusters cannot contain planets. This
conclusion was bolstered in 1999 when Hubble failed to find
close-orbiting "hot Jupiter"-type planets around the stars of the
globular cluster 47 Tucanae. Now, it seems that astronomers were just
looking in the wrong place, and that gas-giant worlds orbiting at
greater distances from their stars could be common in globular clusters.

"Our Hubble measurement offers tantalizing evidence that planet
formation processes are quite robust and efficient at making use of a
small amount of heavier elements. This implies that planet formation
happened very early in the universe," says Steinn Sigurdsson of
Pennsylvania State University.

"This is tremendously encouraging that planets are probably abundant
in globular star clusters," says Harvey Richer of the University of
British Columbia. He bases this conclusion on the fact that a planet
was uncovered in such an unlikely place, orbiting two captured stars 
a helium white dwarf and a rapidly spinning neutron star  near the
crowded core of a globular cluster, where fragile planetary systems
tend to be ripped apart due to gravitational interactions with
neighboring stars.

The story of this planet's discovery began in 1988, when the pulsar,
called PSR B1620-26, was discovered in M4. It is a neutron star
spinning just under 100 times per second and emitting regular radio
pulses like a lighthouse beam. The white dwarf was quickly found
through its effect on the clock-like pulsar, as the two stars orbited
each other twice per year. Sometime later, astronomers noticed further
irregularities in the pulsar that implied that a third object was
orbiting the others. This new object was suspected to be a planet, but
it could also be a brown dwarf or a low-mass star. Debate over its true
identity continued through the 1990s.

Sigurdsson, Richer, and their co-investigators settled the debate by at
last measuring the planet's actual mass through some ingenious celestial
detective work. They had exquisite Hubble data from the mid-1990s, taken
to study white dwarfs in M4. Sifting through these observations, they
were able to detect the white dwarf orbiting the pulsar and measure its
color and temperature. Using evolutionary models computed by Brad Hansen
of the University of California, Los Angeles, the astronomers estimated
the white dwarf's mass. This in turn was compared to the amount of wobble
in the pulsar's signal, allowing the astronomers to calculate the tilt of
the white dwarf's orbit as seen from Earth. When combined with the radio
studies of the wobbling pulsar, this critical piece of evidence told them
the tilt of the planet's orbit, too, and so the precise mass could at last
be known. With a mass of only 2.5 Jupiters, the object is too small to be
a star or brown dwarf, and must instead be a planet.

The planet has had a rough road over the last 13 billion years. When it
was born, it probably orbited its youthful yellow sun at approximately
the same distance Jupiter is from our Sun. The planet survived
blistering ultraviolet radiation, supernova radiation, and shockwaves,
which must have ravaged the young globular cluster in a furious
firestorm of star birth in its early days. Around the time multi-celled
life appeared on Earth, the planet and star were plunging into the core
of M4. In this densely crowded region, the planet and its sun passed
close to an ancient pulsar, formed in a supernova when the cluster was
young, that had its own stellar companion. In a slow-motion
gravitational dance, the sun and planet were captured by the pulsar,
whose original companion was ejected into space and lost. The pulsar,
sun, and planet were themselves flung by gravitational recoil into the
less-dense outer regions of the cluster. Eventually, as the star aged
it ballooned to a red giant and spilled matter onto the pulsar. The
momentum carried with this matter caused the neutron star to "spin-up"
and re-awaken as a millisecond pulsar. Meanwhile, the planet continued
on its leisurely orbit at a distance of about 2 billion miles from the
pair (approximately the same distance Uranus is from our Sun).

It is likely that the planet is a gas giant, without a solid surface
like the Earth. Because it was formed so early in the life of the
universe, it probably doesn't have abundant quantities of elements such
as carbon and oxygen. For these reasons, it is very improbable the
planet would host life. Even if life arose on, for example, a solid
moon orbiting the planet, it is unlikely to have survived the intense
X-ray blast that would have accompanied the spin-up of the pulsar.
Regrettably, it is unlikely that any civilization witnessed and
recorded the dramatic history of this planet, which began at nearly the
beginning of time itself.